Token based xml Parser. Templated to operate with char[], wchar[],
and dchar[] content.

The parser is constructed with some tradeoffs relating to document
integrity. It is generally optimized for well-formed documents, and
currently may read past a document-end for those that are not well
formed. There are various compilation options to enable checks and
balances, depending on how things should be handled. We'll settle
on a common configuration over the next few weeks, but for now all
settings are somewhat experimental. Partly because making some tiny
unrelated change to the code can cause notable throughput changes,
and we need to track that down.

We're not yet clear why these swings are so pronounced (for changes
outside the code path) but they seem to be related to the alignment
of codegen. It could be a cache-line issue, or something else. We'll
figure it out, yet it's interesting that some hardware buttons are
clearly being pushed